Lay Opinion Testimony [FRE 701]

If the witness is not testifying as an expert, the witness' testimony in the form an opinion is limited to one that is

(a) rationally based on witness's perception,
(b) helpful to clearly understand the witness's testimony or the determination a fact in issue, and
(c) not based on scientific, technical, or other specialized knowledge within the scope of Rule 702.

Expert Witness: When Expert Testimony is Appropriate [FRE 702]

A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:

(a) the expert's scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
(b) the testimony is based on sufficient facts or data;
(c) the testimony is the product of reliable principles and methods; and
(d) the expert has reliably applied the principles and methods to the facts of the case.

Expert Witness: Standard used by judges for admissibility [FRE 702]

Expert Witness: Bases of an Expert's Opinion Testimony [FRE 703]: Facts that can be relied on

An expert may base an opinion on facts or data in the case that the expert has been made aware of or personally observed

Includes: (i) outside information learned before trial (e.g., past experience); (ii) information learned for the particular case before trial (e.g., examining car that was in an accident); (iii) can also use facts learned during the trial (big difference between expert witnesses and lay-witnesses)

If experts in the particular field would reasonably rely on those kinds of facts or data in forming an opinion on the subject, they need not be admissible for the opinion to be admitted.

But if the facts or data would otherwise be inadmissible, the proponent of the opinion may disclose them to the jury only if their probative value in helping the jury evaluate the opinion substantially outweighs their prejudicial effect.

Expert Witness: Expert Opinion on Ultimate Issue [FRE 704]

(a) In General — Not Automatically Objectionable. An opinion is not objectionable just because it embraces an ultimate issue.
(b) Exception. In a criminal case, an expert witness must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense. Those matters are for the trier of fact alone.

"Unless the court orders otherwise, an expert may state an opinion — and give the reasons for it — without first testifying to the underlying facts or data. But the expert may be required to disclose those facts or data on cross-examination."

- can the theory be tested (falsifiable)
- has it been subjected to peer review
- what's the error rate on its application
- general acceptance in the relevant community (note: not essential any longer, but if there is general acceptance, that's good evidence that it's reliable)